Autoantibodies targeting proteins at the neuromuscular junction are known to cause several distinct myasthenic syndromes. Recently, autoantibodies targeting neurotransmitter receptors and associated proteins have also emerged as a cause of severe, but potentially treatable, diseases of the CNS. Here, we review the clinical evidence as well as in vitro and in vivo experimental evidence that autoantibodies account for myasthenic syndromes and autoimmune disorders of the CNS by disrupting the functional or structural integrity of synapses. Studying neurological and psychiatric diseases of autoimmune origin may provide new insights into the cellular and circuit mechanisms underlying a broad range of CNS disorders.